1. Field of the Invention
The invention relates to a post-wall waveguide based short slot directional coupler, a Butler matrix using the same, and an automotive radar antenna.
2. Description of the Related Art
In general, a short slot directional coupler comprises waveguides which differ in width from each other and connected in a multi-stepped form. For example, Akihiro Hino, Tadashi Kawai, Yoshihiro Kokubo, and Isao Ohta “A Design of Broad-Band Copact H-Plane Directional Coupler” Department of Electronics, faculty of engineering, Himeji Institute of Technology, MW2000-163, (2000-12) (referred to as Akihiro Hino et al. hereinafter) discloses a short slot directional coupler having a coupling region wherein a wall in common of two arrays of waveguides along the length direction thereof which are arranged in parallel is removed, the waveguides having different widths and connected in a multi-stepped form. The multi-stepped structure allows a wide-banded reflection characteristic to be realized for an arbitrary coupling.
On the other hand, it is known to realize a short slot directional coupler by use of so-called post-wall waveguide (referred to, for example, Shin-ichi YAMAMOTO, Jiro HIROKAWA, Makoto ANDO, “A Beam Switching Slot Array with a 4-Way Butler Matrix Installed in a Single Layer Post-Wall Waveguide,” IEICE Trans. Commun., Vol. E86-B, No. 5, pp. 1653-1659, May 2003 (referred to as Snin-ichi YAMAMOTO et al. hereinafter). The post-wall waveguide can be fabricated by a printed circuit board processing technique because it is formed of a through hole bored in a dielectric substrate. According to Snin-ichi YAMAMOTO, et al., via-holes are arranged in a waveguide axial direction at an interval not more than about 2 times of a diameter of the via-hole so that a leakage is prevented from the interval.
Also, according to the short slot directional coupler of Snin-ichi YAMAMOTO, et al., two arrays of post wall waveguides each comprised of an array of via-holes arranged in line are located in parallel, and so-called narrow wall surface on which both waveguides are in contact with each other is shared to the two arrays of post wall waveguides. A short slot functioning as a coupling element is formed by removing a part of the shared narrow wall. Further, a step for cutting off a TE30 mode and a post for suppressing a reflection are disposed on the area on which the short slot is formed.
A multi-stepped model short slot directional coupler using the waveguide disclosed in Akihiro Hino, et al. is difficult to fabrication, because the waveguides having different widths need to be connected in a multi-stepped form. On the other hand, the short slot directional coupler using the post-wall waveguide disclosed in Snin-ichi YAMAMOTO, et al. needs reflection suppressing posts. These reflection suppressing posts require high location accuracy, because it is in the area where the slot opens and at the position which the coupling is prevented. As a result, its fabrication is difficult in microwave band and millimeter wave band.
It is an object of this invention to provide in easy and low cost a short slot directional coupler which can suppress the reflection without using the reflection suppressing posts requiring high fabrication accuracy.